This invention relates to a fuel cell system including a fuel cell capable of generating electric energy based on chemical reaction of hydrogen and oxygen. This invention is effectively applicable to automotive vehicles, marine vessels, portable power generators and other mobile devices.
When the operation of a fuel cell system is stopped without purging the hydrogen or the oxygen remaining in a fuel cell, the electrode potential of a cathode rises. This induces carbon oxidization reaction and accordingly the cathode electrode extinguishes and deteriorates. Furthermore, reaction gas remaining in a fuel cell may leak via an electrolyte membrane to an opposite electrode. This is a so-called crossleak phenomenon that induces abnormal heat generation and accordingly may damage a catalyst. To prevent this drawback, it is general to carry out purge of the hydrogen or the oxygen remaining in the fuel cell when the operation of the fuel cell system is stopped.
Furthermore, in a fuel cell system installed on an automotive vehicle, it is conventionally known that a discharge resistor is used to consume electric power of the fuel cell when a key switch is switched from an ON position to an OFF position and accordingly the operation of the fuel cell system is stopped, thereby letting the fuel cell system continue power generation so as to remove at least one of the hydrogen and the oxygen. (For example, refer to the Japanese Patent Application Laid-open No. 2001-345114 corresponding to the United States Patent Application Pub. No. US2001/0055705A1)
However, the purge system generally requires providing an inactive gas tank for the purge and accordingly may not be effectively employed for the mobile devices due to difficulty in installation.
According to the fuel cell system disclosed in the above-described Japanese Patent Application Laid-open No. 2001-345114, it may be possible to prevent the cathode electrode from deteriorating when the key switch is switched to the OFF position. However, the feed of electric power from the fuel cell to an electric motor serving as a vehicle driving source is unnecessary when a traveling vehicle is decelerating or the vehicle is stopped to wait for a traffic signal. In such cases, the voltage level of the fuel cell will be close to an open electric potential. This will promote the deterioration.